1. Field of the Invention
This invention relates to an improvement in or relating to a thin film type sulfonation process of alkylbenzenes. More specifically, it relates to a process for preventing the deposition of partially carbonized adherent deposits onto the surface of a reaction wall in a thin film type sulfonation process of alkylbenzenes.
2. Description of the Prior Art
It has been heretofore known in the art, as one process for sulfonating alkylbenzenes, that, while a feed stream of starting alkylbenzenes is flowed downward in the form of a thin film along a surface of a reaction wall of a sulfonation reactor, the thin film stream of the alkylbenzenes is contacted with a gas stream of gaseous sulfur trioxide (SO.sub.3) diluted with inert gas, such as air or nitrogen. This process is widely known as a so-called thin film type sulfonation process and disclosed in, for example, U.S. Pat. Nos. 3,328,460, 3,427,342 and 3,839,391. This process has an advantage in that alkylbenzenes can be continuously sulfonated. However, there is a problem with regard to this process in that partially carbonized adherent deposits are adhered onto the surface of a reaction wall along which the starting alkylbenzenes are flowed downward. The formation or generation of the deposits onto the surface of a reaction wall causes irregularities in the flow of the stream of the thin film, whereby side reactions are accelerated. As a result, the properties of the sulfonated product are markedly impaired. for this reason, in the case were alkylbenzenes are commercially or industrially sulfonated according to the thin film type sulfonation process, there is a disadvantage in that the sulfonation reactor must be inevitably shut down and the surface of a reaction wall thereof cleaned at regular intervals.
It is considered that the formation of the above mentioned deposits on the surface of a reaction wall is caused by the presence of small or slight amounts of impurities contained in the starting alkylbenzenes. That is to say, the alkylbenzenes to be sulfonated are generally produced by the reaction of benzene and alkyl halides in the presence of a FriedelCrafts catalyst, such as, anhydrous aluminum chloride, or by the alkylation of benzene with olefins in the presence of sulfuric acid or a hydrogen fluoride catalyst. However, the alkylbenzenes obtained in these reactions unavoidably contain impurities, such as, catalyst residues, unreacted raw materials, byproducts and the like, although the produced alkylbenzenes are subjected to a purification process, such as rectification. These resultant impurities contained in small or slight amounts in the starting alkylbenzenes are considered to cause the formation of the deposits on the surface of a reaction wall.
In order to obviate the above mentioned problem in the thin film type sulfonation process of alkylbenzenes, it has been proposed, in U.S. Pat. No. 2,806,875, that alkylbenzenes be purified by a series of steps involving sulfuric acid extraction, alkali washing and rectification, in that order. However, based on the inventors' experience, the formation of the deposits onto the surface of a reaction wall cannot be fully prevented by such purification steps.
After intensive studies of the correlation between the impurities contained in the starting alkylbenzenes and the formation of the deposits on the surface of a reaction wall, the present inventors have found that the deposits on the surface of a reaction wall are poly-sulfonated products of olefins contained in the starting alkylbenzenes as impurities. That is to say, the starting alkylbenzenes are generally produced from olefins, or alkyl halides and benzene, as mentioned hereinabove, and the produced alkylbenzenes unavoidably contain at least approximately 100 ppm and up to approximately 2000 ppm of olefins as impurities. As a result, in the case where these alkylbenzenes are used as a starting material in the thin film type sulfonation process, the molar ratio of sulfur trioxide to the olefins becomes remarkably high, as compared with the molar ratio of sulfur trioxide to the alkylbenzenes. In addition, the rate of sulfonation of olefins is faster than that of alkylbenzens. Therefore, the olefin impurities in the alkylbenzenes are sulfonated consecutively in preference to the starting alkylbenzenes at the initial stage of the reaction. Thus, adherent poly-sulfonated products are formed via mono-sulfonated products and adhered onto the surface of a reaction wall.
Accordingly, the formation of the deposits onto the surface of a reaction wall can be prevented in the thin film type sulfonation process of alkylbenzenes when the olefin impurities contained in the starting alkylbenzenes are completely removed. However, the removal of the olefin impurities in an amount of less than 10 ppm from the alkylbenzenes is not practical from an economical point of view.
Various improvements in the sulfonation process in which sulfur trioxide gas is used are known. For instance, U.S. Pat. No. 3,864,375 discloses a process for thin film type sulfonation of olefins or fatty alcohols by the addition of a small amount of a non-ionic surface active agent, whereby the color deterioration of the sulfonated products can be improved and the deposition of the products on the surface of the reaction wall can be prevented. However, we have found that, in the case where alkylbenzenes are sulfonated, this process is not useful.
Furthermore, U.S. Pat. No. 4,113,765 discloses the addition of sulfuric acid or an alkyl benzene sulfonic acid to accelerate the sulfonation reaction in the case where alkylaromatics having a relatively high molecular weight and having a relatively low reactivity are sulfonated in an inert solvent. However, this process is not concerned with a thin-film type sulfonation process. In the case where this process is applied to the thin film type sulfonation of an alkyl benzene, it is expected that the formation of the adherent deposits on the surface of a reaction wall is increased due to the acceleration of the reaction. In fact, we have found that, in the case where an alkyl benzene is sulfonated in thin film type sulfonation by the addition of sulfuric acid, the formation of the undesired deposits on the surface of a reaction wall is increased.
Contrary to the above, we have found that, in the case where an alkyl benzene is sulfonated in thin film type sulfonation by the addition of an alkylbenzene sulfonic acid, no substantial amount of the adherent deposits is unexpectedly formed on the surface of a reaction wall.